Is it really such a terrible investment?

To anyone who knows anything about turntables (or financial transactions in general, I guess):

My friend is selling a turntable she got for Christmas, because she bought the same exact one for herself and her parents unexpectedly got it for her. She paid $150 for it and will give it to me, speakers included for $250.

I mentioned this to my parents and they’re both dismayed. My dad went so far as to call me stupid. I know it’s not the smartest thing to buy something from someone for more than they paid, but it’s a friend and I’m getting speakers out of it, too!

They kept saying you never, never do that, though, especially when it’s used and you don’t get a warranty on it. But it was never even taken out of the box! They’re very adamant about it, which I don’t really understand.

No matter how stupid they think I am, isn’t it my decision? I don’t understand what’s so terrible about this transaction that they have to react so strongly. Maybe I don’t understand, but I didn’t think it was that bad at all to begin with. Could someone help me?